PITTSFORD — Buffalo Bills quarterback Kevin Kolb gave new meaning to the term “freak injury” Saturday when he hurt his knee while slipping – but not falling – on a rubber mat while walking between two practice fields.

“Kevin slipped on his way transferring from field to field,” said coach Doug Marrone. “He bumped his knee and he’ll be day to day.”

Meanwhile, Bills receiver Stevie Johnson estimated he will be out “a week or a few weeks” due to the pulled hamstring he suffered in Friday’s practice.

The net effect of the two injuries is a bonus for the Bills’ top two draft picks – quarterback EJ Manuel and receiver Robert Woods.

Manuel will get even more practice snaps and spend the majority of his time with the first unit for the coming week – or however long Kolb is out with his strained knee.

Woods was already seeing much of his time with the starters but now will get even more work in the key slot-receiver position, which has been Johnson’s primary spot so far.

Kolb’s injury was akin to getting hurt while stepping off a curb or out of the shower.

The Bills had just finished initial walk-through periods and stretching. The quarterbacks and receivers were walking from the grass field, where most of the practice takes place, to the artificial turf field to conduct about 12 minutes of passing and catching. It’s something they do early every practice day.

It’s about a 50-yard walk, and rubber mats cover the asphalt distance between the fields. The mats were a little wet in spots. Kolb slipped.

Bills head trainer Bud Carpenter happened to be walking close by Kolb. Even though Kolb didn’t fall, it was immediately apparent he had “tweaked” something in his left knee. He put his arm on Carpenter’s shoulder then tossed his helmet in frustration before sitting on the ground. He was escorted about 50 yards into the locker room. Workers began sweeping water off the black mats with brooms.

Marrone said the injury is not major.

But Kolb is locked in a battle for the starting QB job, and Manuel arguably has a slight lead through the first week of camp. Any missed time is a setback for the veteran.

Kolb, who turns 29 this month, had trouble staying healthy the previous four seasons. He was knocked out of the Arizona lineup last season by a concussion suffered in a game against the Bills.

Asked about Kolb’s injury history, Marrone said: “Sometimes when I look at players, especially the quarterback position, Kevin has played that position and has taken some big hits. I think anyone in that position probably would have sustained the same amount, give or take a little. It’s disappointing when you get labeled like that because the one thing I do know about Kevin, he is a tough son of a gun. He’ll get back out there quickly if it’s up to him.”

Manuel had all the snaps with the starters Saturday and had 31 plays overall in 11-on-11 work. Jeff Tuel had 27 snaps.

Johnson pulled up with a hamstring injury late in Friday’s practice while running a route over the middle in the end zone.

“Either a week or few weeks,” Johnson said on his recovery time. “It all depends on how I feel, how my leg feels. … I just know it’s pulled. I’ll get in there and get the right treatment so that hopefully I can get back soon.”

Marrone said the team will not rush Johnson back from the injury.

“He’s going to be out for awhile,” Marrone said. “We’re gonna make sure we don’t work hard to get him back as soon as we possibly can. … What I mean by a while is it’s gonna be at least a week and then after that I’ll probably come out with something and see where we are.”

Johnson battled a sore groin last training camp. He has not missed a game the last three seasons.

“Every training camp I’ve had an injury,” Johnson said. “But working with the guys, they’ve always got me out there for Week One and for the rest of the season. Hopefully it’s the same thing. I’m not frustrated really. If it was during the season, I’d be pissed. But right now I don’t want to sound like that player, but I don’t want to force anything and hurt our chances in the season to have me.”

“A lot of times with young receivers, when they come into this league, you’re always talking about have some patience,” Marrone said. “That’s the one thing, it’s a great trait from him. So even pre-draft, when people talked about Robert being NFL-ready, I think that’s what people were talking about. That patience of finding a zone, patience in running the route, not being too quick. The label of he’s NFL-ready, he’s not done anything to disprove that right now.”

Marrone said Woods can play either side of the field as an outside receiver or inside as a slot receiver.

“He’s one of a couple guys that plays all three positions for us, and I think that’s important, especially in a league that’s based on matchups,” Marrone said.

He said Woods, Johnson, rookie Marquise Goodwin and veteran Brad Smith have played all three receiver spots so far.